1. Field of the Invention
This invention relates to a battery check device and more particularly to a battery check device for a camera which is capable of accurately performing a battery check without being affected by temperature.
2. Description of the Related Art
Generally the characteristic of a battery varies with temperature. This has prevented an accurate battery check and presented various inconveniences. For example, a relation between a voltage C.C.V which obtains between terminals when a given load current flows to a lithium battery generally used as a power source for a camera and the number N of shots (weighted differently for flash photography from ordinary photography) each corresponding to a photographing operation is as shown in FIG. 11 of the accompanying drawings with temperature used as a parameter. In FIG. 11, an inhibition level Vcs indicates that the camera is unable to normally operate below this level. As shown, the characteristic of the battery changes in general with variations in temperature According to the results of tests, a maximum photographable number of shots Nmax(L) under a low temperature condition, say, -20.degree. C. greatly differs from a maximum photographable number of shots Nmax(H) under a temperature condition.
Accordingly to the method conventionally employed, a battery check is made as follows: The maximum photographable number of shots Nmax is first obtained from the maximum capacity of the battery. This value is used as an initial value. A number of times which is weighted (for example, differently between the flash photography and ordinary photography) at every shot is subtracted from the initial value. The remainder of the battery capacity is displayed on the basis of the remainder of the shot number. The conventional battery check device of this kind has been arranged to adjust the maximum photographable number of shots Nmax to the maximum shot number Nmax(L) which is guaranteed by the camera using the battery for photographing at the lowest service temperature. Therefore, in the case that photographing is continuously performed at room temperature, further photographing might still be possible to a considerable extent in actuality even when the display indicates nearly zero remainder. Conversely, if the max. shot number Nmax is adjusted to the maximum shot number Nmax(H) photographable at room temperature, continuous photographing at low temperature might result in a failure even when the display indicates a relatively large remaining amount of the battery capacity.
As obvious from FIG. 11, the number of photographable shots (the max. number of shots Nmax) decreases according as the temperature lowers. However, at a point near the inhibition level Vcs, the voltage dropping rate becomes moderate in comparison with the number of shots. Meanwhile, at a relatively high temperature (at room temperature or higher), there is a point at which the terminal voltage C.C.V of the battery becomes almost unvarying irrespectively of the number of shots. Thus, at the relatively high temperature, the number of photographable shots increases. In that event, however, the inclination of the curve comes to increase in the neighborhood of the inhibition level Vcs. Therefore, in addition to the inhibition level, Vcs, if a warning level Vck is set for warning against the impending arrival of the inhibition level Vcs as shown in FIG. 12, the number of shots Nb (Nb1, Nb2, Nb3) photographable after the warning level Vck and before the inhibition level Vcs varies with temperature. Then, the number of shots Nb that can be made after the warning is produced with the battery capacity dropping below the warning level Vck, fluctuates due to temperature.
Further, when the camera is brought into a low temperature condition when the terminal voltage C.C.V. of the battery has lowered to a level close to the warning level Vck after continuous photographing at room temperature or above it, the camera might cease to operate. In other words, the camera suddenly becomes inoperative due to the exhaustion of the battery energy without giving any prior warning. Such an accident results from a change in the battery capacity caused by temperature. For example, when the battery is in a state of having its capacity exhausted 80%, the terminal voltage C.C.V thereof is variable to a greater extent by a change of temperature. Meanwhile, an attempt to solve this problem by setting the above stated warning level Vck at a high level would result in a premature warning under a lower temperature condition and thus would excessively limit the serviceability of the camera.